Impurity elements such as carbon (C), nitrogen (N), oxygen (O), phosphorus (P) and sulfur (S) are known to affect adversely corrosion resistance in alloys. As is also known, reducing to the utmost the content of such impurity elements results in a significant improvement in the corrosion resistance of the alloy.
In mass production of stainless steel in conventional electric furnace-argon oxygen decarburization units (or vacuum oxygen decarburization units), the total content ([C]+[N]+[O]+[P]+[S]) of these impurity elements stood at about 250 ppm, even when performing a removal treatment of the impurity elements by ladle refining.
In vacuum induction melting, by contrast, an alloy ingot is prepared out of a high-purity alloy starting material such as, for instance, electrolytic iron, electrolytic nickel or metallic chromium, using a vacuum induction melter. Therefore, the content of impurity elements can be reduced down to about 10 to 20 ppm for [P] and [S], about 20 to 30 ppm for [N] and [O], and about 30 to 50 ppm for [C]. However, high-purity alloy starting materials are expensive, and hence vacuum induction melting cannot be used for mass production.
Vacuum induction melting employs ordinarily a refractory crucible. Therefore, as is known, it is difficult to reduce the content of impurity elements such as P and N in the melt upon preparation of high-chromium stainless steel. This arises as a result of the fundamental problems below. Removal refining of P in molten steel is ordinarily performed by oxidation refining. In oxidation refining, P in the molten steel is converted to slag-like phosphorus oxide (P2O5) and is removed by being absorbed into the slag. When using oxidation refining to prepare high-chromium stainless steel, however, there is oxidized not only P in the molten steel, but also chromium (Cr), as one alloy component. The Cr content in the steel becomes thereby insufficient.
Thus, a reduction refining technique disclosed in Non-patent Document 1 was developed in the 1970s as a technique for removing impurity elements such as phosphorus (P) in the preparation of high chromium stainless steel. Specifically, melting refining of stainless steel (SUS304) as consumable electrode material is performed in a water-cooled copper crucible, having an inner diameter of φ70 mm, that is provided in an electroslag remelting (ESR) unit, by using CaF2 as fused slag and forming a slag bath by melting metallic calcium into the CaF2. As a result there are removed, for instance, phosphorus (P), tin (Sn), lead (Pb), arsenic (As), antimony (Sb), bismuth (Bi), oxygen (O), sulfur (S), selenium (Se), tellurium (Te), nitrogen (N) and the like as impurity elements in the stainless molten steel. Non-patent Document 1 is the first report on reduction refining using metallic calcium, and is a report on the possibility of removing, in principle, impurity elements such as phosphorus (P) or the like, present in a Cr-containing alloy, by reduction refining. However, ESR processes used in this report requires causing alternate electric current to flow through a slag bath itself, so that the slag bath becomes formed as a result on account of the resulting heat resistance. Therefore, increasing the addition amount of metallic calcium as a way of enhancing the refining effect results in a significant drop in the electric resistance of the slag bath itself, so that a sufficient amount of heat fails to be achieved, and the formation itself of the slag bath is difficult. Thus, the above process was not a practical process.
Ever since, a reduction refining technique using a magnetic-levitation type induction melter (cold-crucible induction melter) provided with a water-cooled copper crucible has been developed, as disclosed in Patent Documents 1 to 3 and Non-patent Document 2. In this refining technique, a melt pool is formed through melting of stainless steel by induction heating, and through addition of a refining agent in the form of metallic calcium and calcium fluoride (CaF2) to the melt pool, to remove thereby impurity elements such as phosphorus (P).
Specifically, a fused calcium fluoride layer is formed first, using calcium fluoride (CaF2) as flux, and then metallic calcium is melted into the fused calcium fluoride layer. The metallic calcium is left to react with the phosphorus (P) in the melt pool, to form calcium phosphide (Ca3P2). The calcium phosphide is absorbed into the calcium fluoride bath. Dephosphorization is carried out thereby. In this refining reaction it is indispensable to use a fused flux such as CaF2 or the like that can melt metallic calcium. Accordingly, there must be used, as the reactor, a water-cooled copper crucible that does not react with molten CaF2 or Ca. That is, this reduction refining technique cannot be used in ordinary vacuum induction melting that employs refractory crucibles.
In this reduction refining technique, removal refining of phosphorus (P) and the like is performed by charging 0.8 to 2 kg of stainless steel (SUS316L), as alloy starting material, into a water-cooled copper crucible having an inner diameter of φ60 mm or an inner diameter of φ84 mm, and by forming a small-scale melt pool. Therefore, the refining techniques disclosed in Patent Documents 1 to 3 and Non-patent Document 2, like the refining technique disclosed in Non-patent Document 1, are proof-of-principle tests in small-scale melt pools, wherein the prepared ingots are merely ingots for research, having at most a weight smaller than 2 kg. To prepare practical-scale ingots of 10 kg or more, therefore, it is necessary to establish anew a reduction refining technique for a large cold-crucible induction melter.
As a large cold crucible-type induction melting method, the inventors established a large-scale cold crucible-type induction melting technique using a water-cooled copper crucible having an inner diameter of φ400 mm or larger, disclosed in Patent Document 4. Upon development of this induction melting technique, however, it was found that the behavior of the melt and slag in a water-cooled copper crucible having an inner diameter of φ200 mm or larger exhibited greater fluctuations as compared with a water-cooled copper crucible having an inner diameter of less than φ100 mm, and found that controlling refining in a large cold-crucible induction melter becomes more difficult the higher the purity of the melt is raised (i.e. the further the content of impurity elements is reduced). As a result, it is unclear whether the reduction refining technique of Patent Documents 1 to 3 can apply, in the production of ultrahigh-purity alloy ingots, also to a melt pool of 10 kg or more, which is deemed to be a practical melt pool. Even assuming that such reduction refining technique can be applied, it is not possible to predict, in the light of the above findings by the inventors, the specific conditions that are necessary for a stable practical-scale operation on the basis of Patent Documents 1 to 3, except in a case where the conditions of reduction refining disclosed in Patent Documents 1 to 3 can be used, without modification, in reduction refining in a large cold-crucible induction melter, or in a case where the reduction refining technique disclosed in Patent Documents 1 to 3 can be optimized to an operation condition of a large cold-crucible induction melter. Therefore, it becomes necessary to establish a separate practical-scale refining technique.
Reduction refining as disclosed in Patent Documents 1 to 3 relies on metallic calcium, such that the Ca content in alloy ingots of stainless steel or the like after reduction refining reaches several hundred ppm. Alloy ingots having been subjected to such reduction refining may be susceptible to impairment of corrosion resistance due to high Ca concentration. Preferably, therefore, Ca is further removed from the melt after reduction refining.
Patent Document 5 discloses a method for producing an ultrahigh-purity alloy ingot by using, as primary ingot, an alloy ingot obtained by performing reduction refining disclosed in Patent Documents 1 to 3, employing a cold-crucible induction melter, and then further removing calcium contained in the primary ingot under an atmospheric pressure lower than 0.5 Pa, using an electron beam melter. As a result there is prepared an ultrahigh-purity alloy ingot satisfying [C]+[N]+[O]+[P]+[S]≦100 ppm and [Ca]≦10 ppm.
Even after performing reduction refining according to the production method of Patent Document 5, however, dephosphorization, decarburization and/or deoxidation were insufficient in some cases, in that [C]+[N]+[O]+[P]+[S]>100 ppm, depending on the amount of metallic calcium and flux, and depending on the operation conditions. That is, no method for producing alloy ingots in a practical scale could be established. Also, electron beam melting must be performed under an ultrahigh vacuum atmosphere, i.e. an atmospheric pressure lower than 0.5 Pa, and hence production costs rise as production time lengthens. Electron beam melting under an atmospheric pressure higher than 0.5 Pa was thus preferable.
When an inexpensive starting material such as stainless steel scrap, carbon steel material, ferrochrome material and the like are used as melt starting materials (alloy starting material) in cold crucible-type induction melting (CCIM), then carbon (C), silicon (Si), manganese (Mn), aluminum (Al) and the like become mixed into the melt, out of the melt starting material, at the melting stage. When using ultrahigh-purity alloy scrap such as ultrahigh-purity stainless steel as the melt starting material, by contrast, virtually no impurity elements as phosphorus (P), sulfur (S), tin (Sn), lead (Pb) and the like become mixed into the melt. However, silicon (Si), aluminum (Al), titanium (Ti), zirconium (Zr), hafnium (Hf), boron (B) and the like do become mixed into the melt, out of the melt starting material. Therefore, it is necessary to perform removal refining of elements such as C, Si, Mn, Al, Ti, Zr and B, derived from melt starting material, according to the target composition of the alloy.
Patent Document 6 discloses a method for removing aluminum as impurity element, that is melted in melt, in cold crucible-type induction melting. Specifically, firstly, melt pool is formed by melting 2 kg of high-Cr ferritic heat-resistant steel (Fe-10Cr), as melt starting material, in a water-cooled copper crucible having an inner diameter of φ84 mm and that is provided in a cold crucible-type levitation melter. Next, 10 g of iron oxide are added to the melt pool, to oxidize the Al that has not melted in the melt, and yield thereby aluminum oxide (nonmetallic inclusion), such as alumina or the like, that does not melt in the melt. Thereafter, 75 g of calcium fluoride (CaF2) as flux are added, to cause the aluminum oxide to be removed through absorption into the CaF2-based flux.
In Patent Document 6, using iron oxide as oxidant of aluminum is effective for removal refining of aluminum since iron oxide is selected as oxide of an element having a weaker affinity to oxygen than that of aluminum. However, as in the examples of Patent Document 6, there can be removed virtually no elements such as carbon (C), silicon (Si), boron (B) and the like having a stronger oxygen affinity than aluminum. Presumably, aluminum alone is removed by iron oxide according to a reaction mechanism that is different from that of the reaction set forth in Patent Document 6. Thus, it is unclear that an element to be removed will be removed even by using an oxidant in the form of an oxide of an element having a weaker oxygen affinity than that of the element to be removed, and it is likewise unclear whether there will be removed an element having a stronger oxygen affinity than the element to be removed, in accordance with the features disclosed in Patent Document 6.
Therefore, this unclear whether Si, Mn and B can be removed down to a target value, even by using the oxidation refining technique of Patent Document 6, in a case where [Si]<0.01 wt %, [Mn]<0.01 wt % and [B]<1 ppm are required in an ultrahigh-purity stainless steel material from which extreme corrosion resistance is demanded. The oxidation refining technique in Patent Document 6 is a proof-of-principle test in a small-scale melt pool that is formed in a water-cooled copper crucible having an inner diameter of φ84 mm, and it is unclear whether the technique applies to a practical-scale melt pool of 10 kg or more. Even if the technique does apply to such a melt pool, the specific oxidation refining conditions required for a stable operation are still unclear.
Patent Document 7 discloses a cold-crucible induction melter that uses a crucible (halide-based crucible) in which a halide layer containing calcium halide, such as calcium fluoride, is formed on the inner side of the crucible, in cold crucible-type induction melting. Although damage to the crucible is suppressed in this cold-crucible induction melter, the reaction progresses at all times through contact between the halide, such as calcium fluoride, and the melt, at an inner wall portion of the halide-based crucible. Accordingly, operational management is more difficult than in the case of using an ordinary water-cooled copper crucible.
As a technique for preparing Ni-based alloy or stainless steel ingots of higher purity, there is an ingot production method by electron beam melting, disclosed in Patent Document 8, that is different from the above-described cold crucible-type induction melting methods. However, electron beam melting is ordinarily used for melting of high-melting point metals such as Ti, Nb and Ta, but methods for removal refining of impurity elements such as carbon (C) and oxygen (O) in stainless steel, by electron beam melting, are still unclear. In particular, specific conditions for stable decarburization and deoxidation refining to [C]≦10 ppm and [O]≦10 ppm are wholly unclear.